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Present understanding of Greenland's subglacial geology is derived mostly from interpolation of geologic mapping of its ice‐free margins and unconstrained by geophysical data. Here we refine the extent of its geologic provinces by synthesizing geophysical constraints on subglacial geology from seismic, gravity, magnetic and topographic data. North of 72°N, no province clearly extends across the whole island, leaving three distinct subglacial regions yet to be reconciled with margin geology. Geophysically coherent anomalies and apparent province boundaries are adjacent to the onset of faster ice flow at both Petermann Glacier and the Northeast Greenland Ice Stream. Separately, based on their subaerial expression, dozens of unusually long, straight and sub‐parallel subglacial valleys cross Greenland's interior and are not yet resolved by current syntheses of its subglacial topography. Plain Language Summary: The Greenland Ice Sheet obscures the rocks beneath 79% of Greenland. By necessity, scientists have relied mostly on studying the rocks exposed along Greenland's edge to understand the island's interior geology. We examine geophysical data from seismometers on the ground, satellites that measure Earth's gravity and magnetic fields and surface topography, and aircraft that measure those same properties and ice thickness. We draw a new map of Greenland's geology beneath the ice sheet by examining where those data show similar signals regarding the nature of the underlying rock, and where they could be related to mapped rock exposures. We also find evidence of some areas with geophysical expressions that are distinct from the rocks found at the island's edges. Some geologic structures, which are entirely covered by ice, may affect how ice flows from Greenland's vast interior toward its coast. Finally, we identify many valleys beneath the ice that are very long and often aligned with each other, but which are not yet fully captured in present maps of the topography beneath the ice. Key Points: We produced a new synthesis of subglacial boundaries for Greenland's geologic provinces from seismic, gravity, magnetic and topography dataThree subglacial regions in central and northern Greenland cannot yet be reconciled with surface‐exposed geologic provincesWe find evidence for a large subglacial valley network that is not fully resolved by subglacial topography syntheses for Greenland [ABSTRACT FROM AUTHOR]

The Greenland Climate Network (GC-Net) consists of 31 automatic weather stations (AWSs) at 30 sites across the Greenland Ice Sheet. The first site was initiated in 1990, and the project has operated almost continuously since 1995 under the leadership of the late Konrad Steffen. The GC-Net AWS measured air temperature, relative humidity, wind speed, atmospheric pressure, downward and reflected shortwave irradiance, net radiation, and ice and firn temperatures. The majority of the GC-Net sites were located in the ice sheet accumulation area (17 AWSs), while 11 AWSs were located in the ablation area, and two sites (three AWSs) were located close to the equilibrium line altitude. Additionally, three AWSs of similar design to the GC-Net AWS were installed by Konrad Steffen's team on the Larsen C ice shelf, Antarctica. After more than 3 decades of operation, the GC-Net AWSs are being decommissioned and replaced by new AWSs operated by the Geological Survey of Denmark and Greenland (GEUS). Therefore, making a reassessment of the historical GC-Net AWS data is necessary. We present a full reprocessing of the historical GC-Net AWS dataset with increased attention to the filtering of erroneous measurements, data correction and derivation of additional variables: continuous surface height, instrument heights, surface albedo, turbulent heat fluxes, and 10 m ice and firn temperatures. This new augmented GC-Net level-1 (L1) AWS dataset is now available at 10.22008/FK2/VVXGUT (Steffen et al., 2023) and will continue to be refined. The processing scripts, latest data and a data user forum are available at https://github.com/GEUS-Glaciology-and-Climate/GC-Net-level-1-data-processing (last access: 30 November 2023). In addition to the AWS data, a comprehensive compilation of valuable metadata is provided: maintenance reports, yearly pictures of the stations and the station positions through time. This unique dataset provides more than 320 station years of high-quality atmospheric data and is available following FAIR (findable, accessible, interoperable, reusable) data and code practices. [ABSTRACT FROM AUTHOR]

The Greenland Climate Network (GC-Net) consists of 31 automatic weather stations (AWS) at 30 sites across the Greenland ice sheet. The first site was initiated in 1990, and the project has operated almost continuously since 1995 under the leadership of the late Prof. Konrad Steffen. The GC-Net AWS measured air temperature, relative humidity, wind speed, atmospheric pressure, downward and reflected shortwave irradiance, net radiation, ice and firn temperatures. The majority of the GC-Net sites were located in the ice sheet accumulation area (17 AWS), while 11 AWS were located in the ablation area and two sites (three AWS) were located close to the equilibrium line altitude. Additionally, three AWS of similar design to the GC-Net AWS were installed by Prof. K. Steffen's team on the Larsen C ice shelf, Antarctica. After more than three decades of operation, the GC-Net AWS are being decommissioned and replaced by new AWS operated by the Geological Survey of Denmark and Greenland (GEUS). Therefore, making a reassessment of the historical GC-Net AWS data is necessary. We present a full reprocessing of the historical GC-Net AWS dataset with increased attention to the filtering of erroneous measurements, data correction, and derivation of additional variables: continuous surface height, instrument heights, surface albedo, turbulent heat fluxes, 10 m ice and firn temperatures. This new augmented G C-Net Level 1 (L1) AWS dataset is now available at https://doi.org/10.22008/FK2/VVXGUT (Steffen et al. 2022) and will continue to be refined. The processing scripts, latest data and a data-user forum are available at https://github.com/GEUS-Glaciology-and-Climate/GC-Net-level-1-data-processing. In addition to the AWS data, a comprehensive compilation of valuable metadata is provided: maintenance reports, yearly pictures of the stations and the station positions through time. This unique dataset provides more than 320 station-years of high quality atmospheric data and is available following FAIR data and code practices. [ABSTRACT FROM AUTHOR]

The basal thermal state (frozen or thawed) of the Greenland Ice Sheet is under-constrained due to few direct measurements, yet knowledge of this state is becoming increasingly important to interpret modern changes in ice flow. The first synthesis of this state relied on inferences from widespread airborne and satellite observations and numerical models, for which most of the underlying datasets have since been updated. Further, new and independent constraints on the basal thermal state have been developed from analysis of basal and englacial reflections observed by airborne radar sounding. Here we synthesize constraints on the Greenland Ice Sheet's basal thermal state from boreholes, thermomechanical ice-flow models that participated in the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6; Coupled Model Intercomparison Project Phase 6), IceBridge BedMachine Greenland v4 bed topography, Making Earth Science Data Records for Use in Research Environments (MEaSUREs) Multi-Year Greenland Ice Sheet Velocity Mosaic v1 and multiple inferences of a thawed bed from airborne radar sounding. Most constraints can only identify where the bed is likely thawed rather than where it is frozen. This revised synthesis of the Greenland likely Basal Thermal State version 2 (GBaTSv2) indicates that 33 % of the ice sheet's bed is likely thawed, 40 % is likely frozen and the remainder (28 %) is too uncertain to specify. The spatial pattern of GBaTSv2 is broadly similar to the previous synthesis, including a scalloped frozen core and thawed outlet-glacier systems. Although the likely basal thermal state of nearly half (46 %) of the ice sheet changed designation, the assigned state changed from likely frozen to likely thawed (or vice versa) for less than 6 % of the ice sheet. This revised synthesis suggests that more of northern Greenland is likely thawed at its bed and conversely that more of southern Greenland is likely frozen, both of which influence interpretation of the ice sheet's present subglacial hydrology and models of its future evolution. The GBaTSv2 dataset, including both code that performed the analysis and the resulting datasets, is freely available at 10.5281/zenodo.6759384 (MacGregor, 2022). [ABSTRACT FROM AUTHOR]

The Greenland ice sheet has been one of the largest sources of sea-level rise since the early 2000s. However, basal melt has not been included explicitly in assessments of ice-sheet mass loss so far. Here, we present the first estimate of the total and regional basal melt produced by the ice sheet and the recent change in basal melt through time. We find that the ice sheet's present basal melt production is 21.4 +4.4/−4.0 Gt per year, and that melt generated by basal friction is responsible for about half of this volume. We estimate that basal melting has increased by 2.9 ± 5.2 Gt during the first decade of the 2000s. As the Arctic warms, we anticipate that basal melt will continue to increase due to faster ice flow and more surface melting thus compounding current mass loss trends, enhancing solid ice discharge, and modifying fjord circulation. Melting at the base of the Greenland Ice Sheet is often disregarded as a source of quantifiable mass loss. In this study, the authors find the basal mass loss is equivalent to 8% of the ice sheet's present imbalance, and that the loss of mass from basal melt is likely to increase in the future. [ABSTRACT FROM AUTHOR]

We present the first 1992–2020 record of Greenland Ice Sheet (GrIS) mass balance derived from multisatellite Ku‐band altimetry. We employ an empirical approach as an alternative detailed to radar‐propagation modeling, and instead convert elevation changes observed by radar altimetry into mass changes using spatiotemporal calibration fields. This calibration field is derived from a machine learning approach that optimizes the prediction of a previously published mass balance field as a function of ice sheet variables. Our mass balance record shows a GrIS contribution of 12.1 ± 2.3 mm sea‐level equivalent since 1992, with more than 80% of this contribution occurring after 2003. Our record also suggests that the 2017 hydrological year is the first year in the 21st century which, within uncertainties, the GrIS was in balance. Overall, the 28‐year radar‐derived mass balance record we present highlights the potential of the method to provide operational mass balance estimates derived from multisatellite Ku‐band altimetry. Plain Language Summary: Earth's land ice masses are shrinking due to climate change. The Greenland Ice Sheet (GrIS) has been a major contributor to sea‐level rise in recent years. Earth‐observing satellites have enabled us to monitor ice‐sheet changes, but common problems for these estimates of ice‐sheet mass balance are mission gaps and changing instrumentation. Since 1991, the European Space Agency has used the same instrumentation for satellite radar altimeters in orbits applicable for monitoring the GrIS. Additionally, before retiring one satellite altimeter, a replacement has been commissioned with a sufficient timespan of overlapping measurements to estimate offsets between satellites. The resulting satellite altimetry record can be processed to provide a lengthy, and consistent, record of ice‐sheet elevation and volume changes. Here, we apply machine learning to convert radar‐derived volume changes to radar‐derived mass changes. This novel mass balance record shows a contribution from the GrIS to global sea level rise of 12.1 ± 2.3 mm since 1992, with more than 80% of this contribution originating after 2003. The method we present is an alternative to modeling both radar propagation and firn compaction, and provides a path toward operational mass‐balance estimates from satellite altimetry. Key Points: Novel estimate of Greenland Ice Sheet mass balance from calibrated multisatellite Ku‐band altimetryTotal eustatic sea‐level rise contribution of 12.1 ± 2.3 mm from the ice sheet since 1992Atmospheric circulation can shift ice‐sheet mass balance by 3σ within 5 years [ABSTRACT FROM AUTHOR]

Mass‐balance estimates of the polar ice sheets presently rely on static ice masks to identify the area under investigation. However, the vintage of available ice masks can differ by more than a decade, and the resolution of the imagery used to delineate ice extent can differ by an order of magnitude, despite evidence of ongoing ice‐sheet retreat. We show that using a fine‐resolution, time‐varying ice mask significantly improves mass‐balance estimates of the northwestern sector of the Greenland Ice Sheet and affects interpretation of local crustal uplift. Accounting for ice margin retreat using a time‐varying ice mask between 2003 and 2015 results in 1.0%–6.8% less ice‐sheet mass loss relative to assuming one of several existing static ice masks. This indicates that mass‐balance estimates that use a static ice mask will progressively overestimate mass loss where the ice sheet is predominantly retreating, meaning a negative bias on mass‐balance. This trend is anticipated to be amplified in a future warming scenario unless updated ice masks are incorporated. For altimetry‐derived mass‐balance estimates, this ice‐mask‐induced bias approaches the magnitude of the ice‐sheet‐wide uncertainty presently associated with spatiotemporal variability in modeled firn compaction. Further, at five Greenland GPS Network (GNET) stations, the ice‐mask‐induced bias is equivalent to 5%–15% of the modeled uplift due to elastic deformation. Given present and projected negative trends in Greenland Ice Sheet mass‐balance and areal extent, we conclude that studies of its mass‐balance should begin incorporating dynamic, periodically updated, fine‐resolution ice masks. This methodological refinement will reduce bias in estimates of both ice‐sheet mass‐balance and crustal uplift. Key Points: Implementing a time‐varying ice mask in northwestern Greenland between 2003 and 2015 decreases inferred mass loss by up to 6.8%This bias approaches the magnitude of ice‐sheet‐wide mass‐balance uncertainty attributed to spatiotemporal variability in firn compactionThe difference in modeled elastic crustal uplift induced by the choice of ice mask is 5%–15% of the modeled uplift at nearby GNET sites [ABSTRACT FROM AUTHOR]

Each summer, surface melting of the margin of the Greenland Ice Sheet exposes a distinctive visible stratigraphy that is related to past variability in subaerial dust deposition across the accumulation zone and subsequent ice flow toward the margin. Here we map this surface stratigraphy along the northern margin of the ice sheet using mosaicked Sentinel-2 multispectral satellite imagery from the end of the 2019 melt season and finer-resolution WorldView-2/3 imagery for smaller regions of interest. We trace three distinct transitions in apparent dust concentration and the top of a darker basal layer. The three dust transitions have been identified previously as representing late-Pleistocene climatic transitions, allowing us to develop a coarse margin chronostratigraphy for northern Greenland. Substantial folding of late-Pleistocene stratigraphy is observed but uncommon. The oldest conformal surface-exposed ice in northern Greenland is likely located adjacent to Warming Land and may be up to ~55 thousand years old. Basal ice is commonly exposed hundreds of metres from the ice margin and may indicate a widespread frozen basal thermal state. We conclude that the ice margin across northern Greenland offers multiple opportunities to recover paleoclimatically distinct ice relative to previously studied regions in southwestern Greenland. [ABSTRACT FROM AUTHOR]

A porous layer of multi-year snow known as firn covers the Greenland-ice-sheet interior. The firn layer buffers the ice-sheet contribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland ice-sheet firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire firn column before interpolating FAC over the entire ice-sheet firn area as an empirical function of long-term mean air temperature (Ta‾) and net snow accumulation (c˙‾). We estimate a total ice-sheet-wide FAC of 26800±1840 km 3 , of which 6500±450 km 3 resides within the uppermost 10 m of firn, for the 2010–2017 period. In the dry snow area (Ta‾≤-19 ∘ C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (Ta‾>-19 ∘ C and c˙‾≤600 mm w.e. yr -1) , FAC has decreased by 23±16 % between 1998–2008 and 2010–2017. This reflects a loss of firn retention capacity of between 150±100 Gt and 540±440 Gt, respectively, from the top 10 m and entire firn column. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and firn retention capacity. [ABSTRACT FROM AUTHOR]

With nine southern Greenland ice sheet ablation area locations, the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) “Q‐transect” is a source of snow accumulation and ice ablation data spanning 17 years (2000 to present). Snow water equivalence measurements below equilibrium line altitude enable resolving the location and magnitude of an orographic precipitation maximum. Snow depth skillfully predicts snow water equivalence in this region, for which we find no evidence of change 2001–2017. After describing observed accumulation and ablation spatiotemporal patterns, we examine surface mass balance (SMB) in 5.5‐km HIRHAM5, 7.5‐km Modèle Atmosphèrique Régional (MAR) v3.7, and 1‐km Regional Atmospheric Climate Model (RACMO2.3p2) regional climate model (RCM) output. HIRHAM5 and RACMO2.3p2 overestimate accumulation below equilibrium line altitude by 2 times. MAR SMB is closer to observations but lacks a distinct orographic peak. RCM ablation underestimation is attributable to overestimated snowfall (HIRHAM5 and RACMO2.3p2), overestimated bare ice albedo (MAR), and underestimation of downward turbulent heat fluxes. Calibrated ablation area RCM SMB data yield −0.3 ± 0.5 Gt/a SMB of the 559‐km2 marine‐terminating Sermilik glacier (September 2000 to October 2012). Using Enderlin et al. (2014, https://doi.org/10.1002/2013GL059010) ice discharge data, Sermilik glacier's total mass balance is −1.3 ± 0.5 Gt/a with interannual variability dominated by SMB. The area specific mass loss is 17 to 20 times greater than the whole ice sheet mass loss after Andersen et al. (2015, https://doi.org/10.1016/j.epsl.2014.10.015) and Colgan et al. (2015, https://doi.org/10.1016/j.rse.2015.06.016), highlighting the Q‐transect's situation in an ice mass loss hot spot. [ABSTRACT FROM AUTHOR]

We present in situ firn temperatures from the extreme 2012 melt season in the southwestern lower accumulation area of the Greenland ice sheet. The upper 2.5 m of snow and firn was temperate during the melt season, when vertical meltwater percolation was inefficient due to a ~5.5 m thick ice layer underlying the temperate firn. Meltwater percolation and refreezing beneath 2.5 m depth only occurred after the melt season. Deviations from temperatures predicted by pure conductivity suggest that meltwater refroze in discrete bands at depths of 2.0–2.5, 5.0–6.0 and 8.0–9.0 m. While we find no indication of meltwater percolation below 9 m depth or complete filling of pore volume above, firn at 10 and 15 m depth was respectively 4.2–4.5°C and 1.7°C higher than in a conductivity-only simulation. Even though meltwater percolation in 2012 was inefficient, firn between 2 and 15 m depth the following winter was on average 4.7°C warmer due to meltwater refreezing. Our observations also suggest that the 2012 firn conditions were preconditioned by two warm summers and ice layer formation in 2010 and 2011. Overall, firn temperatures during the years 2009–13 increased by 0.6°C. [ABSTRACT FROM PUBLISHER]

The article discusses a study from the Geological Survey of Denmark and Greenland (GEUS) glaciology group that focuses on automatic weather stations (AWS) for basic and applied glaciological research. Topics mentioned include a table that shows current sensors used on a standard GEUS AWS, the impact of long polar night and low temperatures on the AWS design, and a discussion on tripod and sonic ranger frame.

The article discusses a study which explored the extreme mass loss of ice sheets based from observations of PROMICE weather stations in Greenland. The study uses atmospheric temperatures and surface solar, known as albedo, which darken the ice-sheets surface. It also suggests that the meltdown may enhance ice mass loss because of ice dynamics such as warming of the ice matrix and basal lubrication.